Infectious diseases are the major cause of morbidity and mortality, accounting for a third of the deaths which occur in the world each year. In addition, infectious agents are directly responsible for at least 15% of new cancers, and they also seem to be involved in the pathophysiology of several chronic diseases (e.g. inflammatory, vascular and degenerative diseases). Traditional infectious diseases are also highly expensive in terms of health-associated costs of infected patients and loss in productivity at work.
The main strategies used to prevent infectious diseases are therapy and prophylaxis. Vaccination has become the most cost-effective measure to prevent infections. However, there are still many diseases for which vaccines are not yet available or the available vaccines are not completely satisfactory due to low efficacy, high reactogenicity, poor stability and/or high costs. Thus, there is still an urgent need for both new and improved vaccines.
Despite the fact that vaccines have traditionally been used for the prophylaxis of infectious diseases, recent findings suggest that they are also a powerful tool for the immunotherapy of transmissible diseases (e.g. viral hepatitis, Helicobacter pylori infections, herpes virus infections, etc.). In addition, vaccines can be used for the immune-therapy or immune-prophylaxis of autoimmune diseases, inflammatory diseases, tumors, allergies and for the control of fertility in human and/or animal populations. In particular, the latter application seems to require the elicitation of efficient mucosal responses at the level of the reproductive tract.
Most infectious diseases are either restricted to the mucosal membranes or the etiologic agents need to transit the mucosa during the early steps of the infection. Therefore, it is desirable to obtain not only a systemic, but also a local mucosal immune response as a result of vaccination, thereby blocking both infection (i.e. colonization) and disease development. This may result in a more efficient protection against infection, facilitating also the eradication of diseases for which humans are the only reservoirs (i.e. blocking transmission to susceptible hosts). Parenterally-administered vaccines mainly stimulate systemic responses, whereas vaccines administered by a mucosal route mimic the immune response elicited by natural infections and can lead to efficient mucosal and systemic responses. Due to the apparent compartmentalization of the systemic and mucosal immune system, parenterally administered vaccines are less effective in protecting against mucosal pathogens (McGhee, J. R., Mestecky, J., Dertzbaugh, M. T., Eldridge, J. H., Hirasawa, M. and Kiyono, H. (1992) The mucosal immune system: from fundamental concepts to vaccine development. Vaccine 10, 75-88). Thus, administration of immunogens through the mucosal route is required to achieve full protection. However, most of the available vaccines are administered through the parenteral route, thereby, eliciting a systemic immunity in the individual.
The administration of vaccines via the mucosal route offers several advantages over parenteral vaccination. These advantages include an ease of administration, the possibility of self-administration (e.g. by intranasal, rectal or oral application), the elimination of the chance of unwanted cross-infection due to the use of infected needles or non-sterile working, lower rates of side effects, higher acceptance by the public, better compliance of vaccination protocols (i.e. increment in the overall efficacy), simpler administration logistics and lower delivery costs, being particularly suitable for mass immunization programs. However, the compartmentalization at the level of the mucosal immune system has to be taken into consideration. In fact, immune responses which can be observed following intra-nasal vaccination may not necessarily occur after oral or intra-rectal immunization. For example, oral vaccination may not stimulate efficient responses in the genitourinary and/or respiratory tracts.
Unfortunately, the delivery of antigens by the mucosal route is associated with a major problem, namely that antigens delivered by this route are generally poorly immunogenic. This is the result of different mechanisms, such as (i) accelerated antigen elimination by the non specific host clearance mechanisms (e.g. ciliar activity, peristaltism), (ii) antigen degradation by local enzymes, (iii) antigen alteration and/or structural modification as a result of extreme pH (e.g. acidic in the stomach, alkaline in the intestine), (iv) poor antigen penetration through the mucosa, (v) limited access of vaccine antigens to antigen presenting cells, and (vi) local peripheral tolerance.
To overcome these problems, different strategies have been used, such as antigen entrapment or association with physical or biological particles (e.g. microparticles, nanoparticles, bacterial ghosts), the use of virosomes or viral-like-particles, the use of liposomes or ISCOMS, the use of transgenic plants, antigen production by attenuated viral or bacterial carriers acting either as conventional vectors or as carriers for nucleic acid vaccines and/or their administration with mucosal adjuvants. However, despite the heavy body of experimental evidence generated in pre-clinical studies during the last years, almost no candidates have been transferred to the vaccine development pipeline.
The use of optimal adjuvants plays a crucial role in vaccination. Antigens administered without adjuvant only rarely mediate an adequate immune response. In addition, not only the strength but also the quality of the elicited immune response matters. Stimulation of an incorrect immunization pattern may lead to immunopathological reactions and exacerbation of the symptoms of infection. In this context, the adjuvant can help to assist the desired immune response. In other words, an adjuvant can modulate the immune response or redirect the immune response to balance the immune response in the desired direction.
Substances referred to as “adjuvants” are those which are added and/or co-formulated in an immunization to the actual antigen (i.e. the substance which provokes the desired immune response) in order to enhance the humoral and/or cell-mediated immune response (“Lexikon der Biochemie und Molekularbiologie”, 1. Band, Spektrum, Akademischer Verlag1995). That is, adjuvants are compounds having immunopotentiating properties, in particular, when co-administered with antigens. The use of many adjuvants is based solely on experience, and the effect can neither be accurately explained nor predicted. The following groups of adjuvants are traditionally used in particular: aluminum hydroxide, emulsions of mineral oils, saponins, detergents, silicon compounds, thiourea, endotoxins of gram-negative bacteria, exotoxins of gram-positive bacteria, killed or attenuated living bacteria or parts thereof.
An overview over the presently known mucosal adjuvants and delivery systems, e.g. the above mentioned particles, ICOMS, liposomes and viral-like particles, for protein-, DNA- and RNA-based vaccines is given in Vajdy et al., Immunol. Cell Biol., 2004, 82, 617- 627. Therein the currently available approaches in immunopentiation of mucosal vaccines are discussed.
That is, various mucosal adjuvants have been described which should serve as an alternative for the adjuvants useful for systemic administration, e.g. see Vajdy et al., supra. These mucosal adjuvants include heat labile enterotoxin and detoxified mutants thereof. In particular, genetically detoxified mutants of heat labile enterotoxin of E. coli have been developed as useful mucosal adjuvants. Moreover, cholera toxin of vibrio cholera is known as an adjuvant useful for mucosal vaccination. Further, the application of unmethylated CpG dinucleotides has been described. It was shown that CpG can bias the immune response towards a Th1 response and can modulate pre-existing immune responses. Saponins are also described as immunomodulatory substances, predominantly via the induction of specific cytokines which then modulate and/or activate the immune response.
In addition, as adjuvants which may be useful in mucosal vaccination the following have been described:
The MALP-2 molecule and Bisaxcyloxypropylcysteine-conjugates thereof, e.g. a Bispalmitoyloxypropylcysteine-PEG molecule is known to represent potent stimulants for macrophages. The usefulness of MALP-2 as an adjuvant was shown previously, see e.g. W02004/009125 and W02003/084568. In particular, it was demonstrated that MALP-2 can act as an effective mucosal adjuvant enhancing the mucosal immune response, e.g. fostering an enhanced expression of antigen-specific IgA antibodies.
Furthermore, it was shown that MALP-2 can activate dendritic cells and B-cells, both play an important role in the induction of a specific humoral immune response. In addition, preliminary studies demonstrate that a combination of biologically active HIV-1 tat protein and synthetic MALP-2 may be a promising vaccine with the MALP-2 component as an effective mucosal adjuvant.
Unfortunately, most of the compounds described above being useful as mucosal adjuvants are not utilizable due to their intrinsic toxicity, e.g. retrograde homing to neuronal tissues of bacterial toxoids and/or toxins at/in the derivatives after nasal vaccination.
Thus, none of these previously described mucosal adjuvants have been approved yet, but, today, only two systemic adjuvants received approval to be administered to humans and, hence, are used for the preparation of human vaccines. These adjuvants are Alum and MF59. However, both are not effective as mucosal adjuvants.
There has been an intensive search in recent years for novel adjuvants, including those for the mucosal administration route. Only a few substances have been found to be able to enhance mucosal responses. Among these, some act as carriers to which the antigens must be bound or fused thereto. Far fewer universally employable “true” adjuvants which are admixed to the antigens have been found, as outlined above.
Prokaryotic as well as eukaryotic cells use various small molecules for cell signaling and intra- and intercellular communication. For example, cyclic nucleotides like cGMP, cAMP, etc are known to have regulatory and initiating activity in pro- and eukaryotic cells. While in eukaryotic cells cAMP and cGMP are used as signaling molecules, prokaryotic cells utilize cyclic di-nucleoside mono phosphate molecules, in particular cyclic diguanosine-mono-phosphate (c-diGMP) beside cAMP.
The condensation of two GTP molecules is catalyst by the enzyme diguanylate cyclase (DGC) to give c-diGMP. C-diGMP has been described as a molecule having anti-microbial activity und may be used to prevent or combat pathogens. Moreover, it was shown that c-diGMP represents one of the key regulators in bacteria. Further, it is known that eukaryotic cells do not use the c-diGMP molecule in its biochemical pathways. In bacterial cells, c-diGMP regulates the expression of genes and the biosynthesis of exo-polysaccharides. Since interacting ligands of c-diGMP are expressed throughout the various genuses of bacteria, it is assumed that most bacteria use c-diGMP as a regulatory molecule.
In WO 2005/087238, it has been speculated that cyclic diGMP or analogs thereof can stimulate or enhance immune or inflammatory response in a patient or can enhance the immune response to a vaccine by serving as an adjuvant. Further, it is speculated that cyclic diGMP or its analogs may be used as active ingredient in compositions for treating injuries, diseases, disorders and conditions that result in neurodegeneration. Therein, data are provided showing that cyclic diGMP does not modulate DC endocytic activity but may activate dendritic cells due to induction of expression of co-stimulatory molecules. Further, data are provided showing that occasionally c-diGMP may upregulate immunostimulatory capacity of dendritic cells. Further, data are provided showing that c-diGMP in high doses may activate T-cells in vitro when mixed with dendritic cells. However, any enhancement of immune or inflammatory responses in a patient or enhancement of the immune response to a vaccine by serving as an adjuvant is not shown, rather it is speculated therein that there are some data which may indicate for an increased presentation of antigen through stimulation of HLA-DR. Further, no immunomodulatory action of cyclic diGMP is shown in said document. Hence, this document merely speculates about any immunomodulatory, in particular, about any enhanced immune response by serving as an adjuvant. As discussed before, an adjuvant is a compound able to provoke or enhance the humoral and/or cell mediated immune response against an active antigen. No data are provided in WO 2005/087238 showing an immune response against an active antigen using c-diGMP as adjuvant for enhancing or eliciting or modulating said immune response. In addition, it is noted that said document only provides information regarding c-diGMP but not with respect to any other analogs of cyclic diGMP.
Hence, there is still a need in the prior art to provide new compounds useful as adjuvants, particularly as mucosal adjuvants and/or as vaccines. In particular, there is a need for mucosal adjuvants which can elicit a strong immune response which represent a balanced or adjusted immune response involving both humoral and cellular components, thus, allowing effective prophylaxis or treatment of various diseases and conditions, specifically of infectious diseases or cancer.
Thus, the object of the present invention is the provision of mucosal adjuvants which can elicit and/or enhance and/or modulate (pre-existing) immune response in an individual or subject. In particular, the invention was based on the object of developing a range of novel, highly active adjuvants, particularly mucosal adjuvants which are non-toxic for humans and which can be employed with a wide variety of active ingredients to be assisted in conventional or novel vaccines such as, in particular, prophylactic or therapeutic vaccines, including cancer and DNA vaccines.